Recently, development of a thin film transistor using an organic material which exhibits a semiconducting property (that is, an organic semiconductor material) is advanced.
This thin film transistor is suitable for being made thinner and for reducing weight, has flexibility and can be manufactured at a low material cost. Such a thin film transistor having the above advantages is expected to be used as switching devices of a flexible display.
It is known that an inorganic material is often used as a constituent material of, a gate insulating layer of such a thin film transistor (see, for example, patent document 1: JP-A 2004-103719).
However, in the thin film transistor, the gate insulating layer is formed using a vapor phase film forming method. Therefore, there is a problem in that the formation of the gate insulating layer becomes complex and takes a long period of time. Further, there is also a fear that an organic semiconductor layer of the thin film, transistor is alternated or deteriorated during the formation of the gate insulating layer.
In order to resolve the above disadvantages, as the constituent material of the gate insulating layer, for example, a phenol-based resin, an acryl-based resin or the like is often used.
On the contrary, since these resins have polarity, they exhibit high hygroscopicity. In the case where a thin film transistor having the gate insulating layer constituted from such a resin material is used in the air, the gate insulating layer absorbs moisture. When an amount of water contained in the gate insulating layer is increased, ion current begins to flow through the gate insulating layer. This causes problems such as shift of threshold voltage (Vth) of the thin film transistor, increase of gate leak current and easy occurrence of dielectric breakdown.